Prior art devices for holding wafers during processing include a spring-loaded clamping ring as shown, for example, in Faretra, U.S. Pat. No. 4,282,924, which is assigned to the assignee of the present invention. This clamping apparatus has the disadvantage that the clamping ring presses against the surface of the semiconductor wafer being held in position by the clamping apparatus, thus wasting valuable silicon surface area which is not available for processing. Since such a clamping ring is adjacent to the wafer surface, sputter contamination of the surface of the wafer during ion implantation may result from energetic ions striking the clamping ring. Further, the sliding surfaces and rubbing springs of such devices tend to generate particles which may contaminate the wafer surface.
In commercially available batch processing ion implantation systems, a plurality of platens is arranged concentrically around the periphery of a disk, and wafers held on the platens are implanted as the disk rotates. In these systems, the devices used to hold wafers on the platens against the centrifugal force generated by the rotation of the disk include an arcuate bumper on the outer edge of each platen and a spring driven mechanism to slide the wafers against the bumper. The sliding of a wafer across the platen tends to generate particles on the lower surface of the wafer. Such particles impede thermal contact between the wafer and the platen and may be transported to contaminate the wafer surface.
The wafer holding apparatus of the present invention avoids these negative features of prior art holding devices.